There are two major types of field effect transistors or FET's: the metal-oxide-semiconductor field effect transistor or MOSFET (also called an insulated-gate FET, or IGFET), and the junction-gate FET, or JFET. An FET has a control gate and source and drain regions formed in a substrate. The control gate is formed above a dielectric insulator that is deposited over the area between the source and drain regions. As voltage is applied to the control gate, mobile charged particles in the substrate form a conduction channel in the region between the source and drain regions. Once the channel forms, the transistor turns “on” and current may flow-between the source and drain regions.
FIGS. 1 and 2 illustrate the structures of two main types of conventional thin-film transistors. As illustrated in FIG. 1, one type is a bottom contact structure 10. Structure 10 has a control gate electrode 6 formed above a dielectric insulator 1. Insulator 1 is formed over a semiconductor 2. In turn, semiconductor 2 is formed over a substrate 3. A source electrode 4 and a drain electrode 5 are formed in substrate 3. Gate electrode 6 is deposited over the area between source electrode 4 and drain electrode 5.
As illustrated in FIG. 2, a second type of conventional transistor is a top contact structure 20. Structure 20 has a control gate electrode 6 formed in or on the substrate 3. Insulator 1 is formed over substrate 3 and gate electrode 6. In turn, semiconductor 2 is formed over insulator 1. A source electrode 4 and a drain electrode 5 are formed on top of semiconductor 2. Gate electrode 6 is formed under the area between source electrode 4 and drain electrode 5.
Transistors are used as either amplifying or switching devices in electronic circuits. In the first application, the transistor functions to amplify small ac signals. In the second application, a small current is used to switch the transistor between an “on” state and an “off” state.
The bipolar transistor is an electronic device with two p-n junctions in close proximity. The bipolar transistor has three device regions: an emitter, a collector, and base disposed between the emitter and the collector. Ideally, the two p-n junctions (the emitter-base and collector-base junctions) are in a single layer of semiconductor material separated by a specific distance. Modulation of the current flow in one p-n junction by changing the bias of the nearby junction is called “bipolar-transistor action.”
External leads can be attached to each of the three regions and external voltages and currents can be applied to the device using these leads. If the emitter and collector are doped n-type and the base is doped p-type, the device is an “npn” transistor. Alternatively, if the opposite doping configuration is used, the device is a “pnp” transistor. Because the mobility of minority carriers (i.e., electrons) in the base region of npn transistors is higher than that of holes in the base of pnp transistors, higher-frequency operation and higher-speed performances can be obtained with npn devices. Therefore, npn transistors comprise the majority of bipolar transistors used to build integrated circuits.
An insulated-gate bipolar transistor or “IGBT” is used to control the flow of electric power. It contains four regions of semiconductor material of alternate conductivity type, and it has three external terminals. The device controls the flow of power in response to a signal applied to one of its terminals, called the gate. The presence of an appropriate gate signal turns the device on and allows electric current to flow through it; removing the gate signal turns the device off, blocking the flow of current.
A “thyristor” is a semiconductor device similar to an IGBT. Like an IGBT, a thyristor contains four regions of semiconductor material of alternate conductivity type and has three external terminals, one of which is the gate. As in the IGBT, applying an appropriate gate signal turns the thyristor on, allowing the flow of electric current through it. Unlike an IGBT, however, removing the gate signal from a thyristor does not shut off the flow of electric current through the device. Once a thyristor turns on in response to the application of a gate signal, it cannot be turned off simply by removing the gate signal. The thyristor thus exhibits “latching” behavior. In response to the application of an appropriate gate signal, the device turns on and remains on even if the gate control signal is removed. Turning a thyristor off typically requires reduction of the current flowing through the device below a threshold level.
The latching property of the thyristor arises from the structure of the device. The four alternating semiconductor regions in a thyristor inherently incorporate two three-layer combinations, each of which has a forward current gain, denoted as σ1 and σ2, respectively. It is well known that a thyristor will not latch if the sum of σ1 and σ2 is less than one.
A MOSFET is also a three-terminal device that is used to control the flow of electric power. Unlike IGBT's and thyristors, however, MOSFETs have only three semiconductor regions. A MOSFET controls the flow of power through the device in response to an appropriate control signal applied to its gate terminal. MOSFETs are similar to IGBTs in that they can be used to control the flow of electric power by selectively applying and removing an appropriate gate signal. MOSFETs do not exhibit the “latching” behavior of thyristors, but thyristors can typically carry larger amounts of electric power.
IGBTs combine the controllability of a MOSFET with the high-power-carrying capabilities of a thyristor. Because they incorporate a four-layer structure similar to a thyristor, however, IGBTs incorporate two three-layer combinations of regions of alternate conductivity and therefore exhibit latching if subjected to certain electrical conditions, such as high voltages.
The semiconductor-based devices and systems described above conventionally incorporate inorganic semiconductor materials, for example, silicon-based materials. Organic semiconductors have the potential to replace conventional inorganic semiconductors in a number of applications, and further may provide additional applications to which inorganic semiconductors have not been utilized. Such applications may include, for example, display systems, mobile devices, sensor systems, computing devices, signal reception devices, signal transmission devices, and memory devices.
Replacement in these applications is anticipated because organic semiconductors have advantages when compared to conventional semiconductors. One important advantage is the relative ease of processing organic semiconductors. Another advantage is the improvement in electrical properties possible using organic semiconductors. The electrical properties of organic semiconductors depend largely on their intrinsic material properties, morphology, crystallinity, and the packing density of molecules.
More specifically, for organic thin-film transistors, the interfaces between components have played an important role. The interface between the gate insulator and the organic semiconductor determines transistor stability: the cleaner the interface, the more stable is the thin-film transistor. Currently, the organic semiconductor and the insulator layers are deposited separately by using vacuum deposition, spin coating, ink-jet printing, and the like. Therefore, the chances of contamination and imperfection at the interface are high, rendering current processes unlikely to generate a high yield of reliable devices.
In addition, the structure of the transistor plays another important role in defining the electrical properties of the device. Conventional transistor structures have a gate consisting of a gate insulator and a gate electrode located on only one side of the semiconductor. This restricts transistor properties.
The semiconductor industry has recently begun to recognize the importance of multi-gate semiconductor structures—although using conventional silicon-based materials. See, e.g., M. Masahara and E. Suzuki, “AIST Announces World's Thinnest Vertical-Type Double-Gate MOSFET Using Newly Discovered Process,” AIST Today Int'l Ed. No. 8, pages 10–12 (2003) (incorporated in this document by reference). The multi-gate structure has more than one gate around the semiconductor. The structure allows optimal control of transistor properties, especially of the threshold voltage, and both minimizes power consumption and reduces switching error. More specifically, less drain current is required to turn on the device and, therefore, less power is consumed while the device is operating. In addition, each of the multiple gates can be used to control the gate threshold voltage of another gate, which allows optimal control of the threshold voltage.
The vertical-type, double-gate MOSFET considered by authors M. Masahara and E. Suzuki is designed to eliminate the short-channel effect, which is the mutual interference between the source and the drain as the distance between those components is reduced through miniaturization, by layering a thin channel between the two gates. As the authors recognize, however, fabrication of multi-gate structures has proven difficult. The authors disclose fabrication of the world's thinnest channel (about 15 nm thick) by using a conventional complementary metal-oxide-semiconductor (CMOS) production process in combination with a newly discovered process in which the etching rate of an alkaline solution is retarded at the surface exposed to ion bombardment.
To overcome the shortcomings of conventional semiconductor devices and manufacturing processes, a new device and process of manufacture are provided. An object of the present invention is to provide an improved combination insulator and organic semiconductor. A related object is to reduce the possibility of an imperfect or contaminated interface between the insulator and the organic semiconductor. Another object is to reduce the number of steps during the process of manufacturing a combination insulator and organic semiconductor.
It is still another object of the present invention to provide an organic, thin-film transistor having improved electrical properties. A related object is to provide an organic, multi-gate, thin-film transistor. More specific objects are to allow optimal control of transistor properties, especially of the threshold voltage, and to both minimize power consumption and reduce switching error. Yet another object is to provide a process facilitating the manufacture of multi-gate transistor structures.